Preferably, the above-mentioned apparatus is applied in an electrical actuator for open and/or closed loop control of a flow regulating device. The flow regulating device is, for example, a valve, a gate, a throttle or a flap. Depending on the flow regulating device to be actuated, the movement performed by the actuator is a rotation or a translation, in given cases, a rotation of less than 360 degrees. The invention is not limited to the embodiment of an actuator. Rather, it can be used in any case where a traveling nut runs against a stop and then subsequently must be capable of being easily released, i.e. backed away, therefrom.
In actuators, the torque transmission between an electric motor, or, in general, an actuating element, and the flow regulating device occurs by way of a speed reduction transmission, which, depending on the case of application, can be a bevel gear, or spur gear, transmission, a worm gear transmission, a superimposed transmission, or a lever transmission. The speed reduction transmission is necessary, in order to convert the high RPM of the electric motor into the desired, in high measure constant, output RPM for actuating the flow regulating device. The assignee manufactures actuators meeting the most varied of requirements. Thus, torque in the case of rotary drives can reach to 32,000 Nm; in the case of rotary drives with less than 360 degree range, torques can be implemented at up to 360,000 Nm.
Used for reducing the RPM of the electric motor down to the output RPM, with which the flow regulating device is to be actuated, is, for example, a worm gear transmission with worm shaft, worm and engaging worm wheel, in connection with a planetary gear transmission. In order to assure that the worm gear transmission remains in the desired rest position in the case of shutdown of the electric motor, the worm gear transmission includes a self-locking feature. Worm gear shaft and output hollow shaft with worm wheel rotate usually in ball-, or dry bearings.
The worm is arranged on the worm gear shaft shiftably between two measuring spring packages, so that the worm, in the case of a torque to be transferred, experiences a translational movement relative to the worm gear shaft. This shifting, which is a measure for the torque being transferred, is forwarded to a control unit. The interior of the transmission is filled with lubricant, so that maintenance free operation is assured over a longer period of time.
Depending on the type of construction of the flow regulating device, the rotary drive must be turned off in the end positions path, or torque, dependently. Provided for this in the control unit are usually two independent measuring systems, namely a path circuit and a torque circuit, which measure, respectively, traveled actuation path distance and the torque applied on the output shaft. The reaching of a desired position is sensed by the control unit via a switch, whereupon the control unit turns the electric motor off.
In order to prevent damage to the transmission or the flow regulating device in the case of malfunction and, thus, in the case of travel beyond one of the end positions, two end stops are provided, which limit the travel of the nut. However, the coming of the nut into contact with one of the end stops occurs with high torque, so that the release torque involved with getting the nut back off the end stop is correspondingly high. Making this more difficult is that grease in the contact between end stop and nut exudes away over time, whereby the release torque compared with the tightening torque even increases. This leads to considerable problems in the case of restarting a stuck actuator following a malfunction. For the purpose of reducing the release torque, in the case of known solutions of the assignee, the bearing area of the nut is, therefore, rounded, i.e. given a convex shape.